This invention relates to an improved liquid phase process for preparing 1,1-dichloro-2,2,2-trifluoroethane (xe2x80x9cHCFC-123xe2x80x9d or xe2x80x9c123xe2x80x9d) by the catalyzed fluorination of perchioroethylene (xe2x80x9cPERCxe2x80x9d) in the presence of hydrogen fluoride (xe2x80x9cHFxe2x80x9d), particularly to a process which involves the use of a catalyst consisting essentially of an antimony (V) catalyst. HCFC-123 is particularly useful as an intermediate for making pentafluoroethane (xe2x80x9cHFC-125xe2x80x9d or xe2x80x9c125xe2x80x9d), a known refrigerant.
U.S. Pat. No. 6,049,016 also discloses a liquid phase process for preparing 123 by the catalyzed fluorination of PERC in the presence of HF, but in its example using an antimony catalyst, a mixture of SbF3 and SbF5, a substantial amount of the olefin 1112a (CC12=CF2) was produced. Since the boiling point of this olefin (19xc2x0 C.) and 123 (27xc2x0 C.) are close, separation by distillation can be expected to be difficult and require an expensive chemical treatment such as adsorption or conversion of the 1112a to a higher boiling compound. If the olefin is not separated, presence of the olefin can be expected to result in faster deactivation of catalyst used when the 123 is converted to 125. Thus, it would be useful to have a process which avoids the formation of olefin impurities.
An improved liquid phase process for preparing 123 by the catalyzed fluorination of PERC in the presence of HF is provided, which improvement comprises using a catalyst consisting essentially of an antimony (V) catalyst such as SbF5. An unsupported or supported catalyst may be used, the support typically being carbon. Because the catalyst is corrosive in the presence of HF, a preferred embodiment involves the use of a reactor made out of or lined with a fluororesin such as polytetrafluoroethylene (xe2x80x9cPTFExe2x80x9d). Particularly preferred is the use of an adiabatic fluororesin reactor since the heat liberated by the exothermic PERC to 123 reaction will supply most of the heat of rectification to remove the 123 and by-product HCl as a vapor from the system. The 123 can then be readily converted to 125 by known processes, such as by fluorinating the 123 with HF in a gas phase process in the presence of a fluorination catalyst such as a supported or unsupported Cr(III)-based catalyst.
It has now been found that use of an antimony (V) catalyst in the liquid phase fluorination of PERC to 123 avoids formation of olefin impurities such as 1112a. The higher the fluorine content of the Sb (V) catalyst the better, SbF4Cl or, preferably, SbF5 thus being the most active species. The catalyst may be unsupported or it may be used on a support such as carbon, graphite, alumina, or fluorided alumina.
The process of this invention can be conducted as a batch or continuous process. Inorganic solvent, such as excess HF, or organic solvent, such as excess PERC or a hydrochlorofluorocarbon (xe2x80x9cHCFCxe2x80x9d) such as 123, 1,1,2-trichloro-2,2-difluoroethane (xe2x80x9c122xe2x80x9d) or 1,1,2,2-tetrachloro-2-fluoroethane (xe2x80x9c121xe2x80x9d), or mixtures of such solvents may also be present. Thus, in a continuous reaction, 122 and 121 by-products can be fed back to the reactor. The HF:PERC molar ratio is typically from 1 to 50, preferably from 2 to 10. The temperature is typically from 20 to 300xc2x0 C., preferably from 50 to 200xc2x0 C. The pressure is typically from about 50 to 600 psig, preferably from 100 to 300 psig. The by-product HCl can be removed from the resultant reaction mixture by methods known in the art such as by absorption (in water or caustic) or by distillation. The HCl can also be continuously removed from the reactor during the reaction by distillation.
To convert 123 to 125 in a gas phase fluorination reaction, 123 and excess HF are typically reacted in the presence of an oxygen-containing gas such as air, to maintain catalyst activity, and a Cr (III)-based catalyst such as chromium oxide. A cocatalyst such as Ni (II), Co (II), Zn (II) or the like may also be used. Typical operating temperatures and pressures are 200-400xc2x0 C. and 1-10 atmospheres (preferably 1-6 atmospheres). The molar ratio of HF to 123 is typically 2-10.